1. Technical Field
The present invention relates in general to wireless communication systems and in particular to wireless systems utilizing Code Division Multiple Access ("CDMA"). More particularly, the present invention relates to maximizing sector capacity of a given system by improving call drop probability. Still more particularly, the present invention relates to improving soft handoff.
2. Description of the Related Art
CDMA, as specified by CDMA standard IS95, is a form of digital cellular phone service and generally offers increased capacity over other types of digital cellular phone service. Each phone call is combined with a code that is broadcast across a broad frequency spectrum and another phone, which is aware of the code, receives the signal among all the other signals that may be occupying that frequency band. By coding the signal so that only one phone may receive the signal, more transmissions on the same band are allowable. Each cell may be partitioned, by directional antennas, into a number of sectors. Sectors may have different pilot signals and users and resources are shared by the multiple sectors within a cell.
CDMA power requirements represent both an advantage and a disadvantage to the system. In CDMA, strong signals overpower weak signals because the noise level is raised at the base station demodulators to accommodate the strong signal. The noise level problem is somewhat overcome by power control. The base station samples signal strength indicators of each mobile and sends a power change command to the mobiles increasing or decreasing the power requirement as a function of the grade of service requirement. This causes a nearby, strong mobile to decrease its power output and a mobile with a weak signal to increase its power output.
CDMA also provides a feature called "soft handoff." "Hard handoff," as opposed to soft handoff, is the process a wireless phone (mobile or handset) goes through as it approaches the boundary of a new cell. The network automatically drops resources in the current cell and hands off the connection to the new cell the mobile may be entering. Soft handoff allows a mobile to maintain resource connections with multiple base stations while moving within a system, adding and dropping connections as necessary.
IS95 (digital CDMA standard for U.S. cellular radio systems) soft handoff allows an individual handset to maintain a connection with as many as six individual pilot signals. As a mobile demodulates received information and sends modulated information, the mobile is constantly searching for pilot signals. A pilot signal (identifier channel, designated P1, P2, P3, etc.) broadcast from each sector of each base station (fixed station for communication between a network and mobiles within base station cells), is unique to that sector and is identified with a unique code--a PSEUDO NOISE ("PN") sequence. If a handset ("mobile") detects a new pilot, not yet in communication with the mobile, whose pilot strength (carrier to total interference ratio) is above an upper signal strength threshold (T_ADD), it will send a Pilot Strength Measurement Message ("PSMM") to the network via the sector(s) base station with which it is currently communicating. The PSMM is sent to request that this new sector be added to the mobile's "active set" (a set, on board the mobile and network containing sectors that are currently in communication with the mobile). The network will instruct the mobile to add this new pilot via a Handoff Direction Message (HDM) sent out by all the sectors in the mobile's current active set. The HDM includes parameter settings based on changes in signal strengths, number of pilots in an active set and new parameter values introduced by the system operator. The mobile, on receiving the message, will add this new sector to the active set utilizing the parameters provided in the HDM and acknowledge via a third message, the Handoff Completion Message (HCM). If the mobile detects that a current, active set pilot signal strength (carrier to total received signal ratio) has dropped below a certain lower signal strength threshold (T_DROP) and has remained below that threshold for a pre-determined period of time (T_TDROP), then a PSMM is sent to the network, requesting that such a sector be dropped from the active set. The HDM and HCM follow in order as explained above. Upon receiving the HCM, the network acknowledges the HCM by sending a BSAO (Base Station Acknowledgment Order).
If a PSMM is sent by a mobile requesting addition of a particular pilot into the mobile's active set, the network may choose not to add such a Pilot. In order to reduce excessive messaging back and forth, the network may not send an HDM because the mobile would just resend the PSMM. However, the network must acknowledge the PSMM to prevent the mobile from continuously sending the PSMM. In such cases, the network acknowledges the PSMM with a Base Station Acknowledgment Order (BSAO), but doesn't send the HDM. This action prevents the mobile (standard action) from sending a PSMM requesting addition of this particular pilot again, until the pilot exceeds the strength of the current, weakest active set pilot by a factor "T_COMP" (T_COMP is an assigned value for triggering a decision by the network to add a new channel to the active set. New pilot signals must exceed the weakest active set pilot signal plus T_COMP. The IS95 standard requires T_COMP to take on positive values.). As the new pilot exceeds T_COMP (plus weakest signal), a completely new PSMM will be triggered. If the message is again ignored (BSAO sent, not HDM), then another PSMM will be sent only if this particular pilot exceeds the weakest pilot in the active set by T_COMP. Keep in mind that should anything else change, i.e. another pilot needing entry to the active set or a current active set ready to drop a pilot, a subsequent PSMM will be sent.
In an active set, as indicated before, there may be as many as six sectors allocating resources for an individual handset. For example, a mobile may have an active set of P1, P2, P3, and a new pilot, i.e., P4 may be increasing in strength so much so (mobile is in motion within cells causing pilots in an active set to change strength) that P4 equals or exceeds P3. Consequently P4's signal strength generates interference with P3. T_COMP should be set so that a PSMM is triggered when the signal to noise ratio is such that the mobile would be dropped from the system because of too much interference. Another way to add a new pilot is that if a current, active set pilot, P3, drops below T_Drop (a system specified arbitrary value) for a specified time period (T_TDROP), usually four seconds, a PSMM is triggered. The PSMM requests the removal of the sector from the active set because the pilot C/I (carrier to total interference ratio) dropped below T_Drop for T_TDrop seconds. The HDM and HCM follow in order as explained above
Referring now to FIG. 4A, a graph illustrating signal strength of P3, as received by a mobile, is illustrated. P3400 is a pilot signal increasing in strength as the mobile moves within the system. An active set (not shown), consisting of P1 and P2, will be considering P3400 as a candidate for the active set. As the mobile moves closer to P3, T_ADD 402 is reached (-6 db from P2, the weaker signal in the active set) and a PSMM is sent to a base station. In this instance, P3 is not added because Delta.sub.-- 3 (arbitrary value for three pilots in an active set, set by the system) condition is not met, and P3 continues to increase in strength to -8 db which is the signal strength of P2. If the mobile continues moving towards P3, the next PSMM to be sent will be PSMM 404. The signal strength of P3 when PSMM 404 is sent, meets or exceeds T_COMP 406 (including weakest signal strength) which is the trigger for PSMM 404.
Referring to FIG. 4B, a chart illustrating constants required to add and drop pilot signals to an active set, is depicted. As previously discussed, a signal increasing in strength passes through certain thresholds that permit addition of a pilot signal to an active set and also permit dropping a pilot from the active set. T_ADD 410 is reached by P3 and P3 is added to the active set. After a period of decreasing signal strength, the signal strength of P3 falls through T_DROP 412. T_DROP 412 threshold is a trigger point that starts measuring the period of time P3 remains below T_DROP 412. If the signal remains below T_DROP 412 a predetermined period of time (usually 4 seconds), T_TDROP 414 causes the signal to be dropped from the mobile's active set.
A mobile may demodulate a maximum number of received transmission paths, usually three. Consequently, the mobile will attempt to demodulate the three highest quality paths from any of the links at any instant in time (this changes as radio frequency conditions change). By demodulating three channels simultaneously, probability increases that signal reception is clear and soft handoff would maintain continuity when moving between cells. A problem with high handoff rate is that system capacity is sacrificed, as there are sectors having to transmit (including those in the active set) even though the mobile is demodulating from other sectors. Optimal handoff is required to maintain frame error rates (invalid frame, or packet, identified by the Cyclic Redundancy Check sent with each frame) and dropped call (mobile permanently disconnected from the system during communication) probabilities below selected targets by providing a diversity of signal paths to the mobile.
All base stations are expending power transmitting to mobiles that are active within the system. Where a mobile active set contains six pilot channels, there are six sectors expending power maintaining radio connection with the mobile. Therefore, it would be desirable to increase efficiency of the network by providing a method and apparatus for assisting soft handoff between a mobile (or sectors) and base-stations while maintaining low dropped call probabilities and without adversely affecting frame error rates.